Biological systems, such as humans, animals, plants and microorganisms, require inorganic elements (trace elements, macroelements) and minerals for physiological processes and for the synthesis of active substances. The nutrient sources available for such biological systems provide necessary elements and minerals in very different amounts and chemical modifications. It may therefore be necessary to make available to biological systems additional nutrients with inorganic elements and minerals in suitable quantities and in utilizable forms.
According to various studies, more than 30 trace elements are regarded as essential for vital processes. The trace elements include a large proportion of metals, such as e.g. iron, copper, manganese and zinc, but also some non-metals, such as e.g. iodine, selenium, bromine and fluorine.
Trace elements which are absent from the diet of humans, animals, plants and microorganisms can lead to deficiency symptoms, toxic actions or reduced yield, for example in microbial processes, such as biofermentation, or in plant and animal production.
Essential factors for the availability of nutrients are, in addition to the uptake capacity of a biological system, inter alia the quantities or concentrations offered to the biological system and the forms in which the nutrients are provided to the biological system. The latter forms of nutrients include not only the compounds themselves in which the nutrients are present, but often also the ambient conditions under which the nutrients are offered to the biological system, for example the pH.
The availability of nutrients in the soil to plants is influenced by various factors. Thus, for example, the trace elements B, Cu and Zn have an optimum availability to plants at a pH in the soil of between 5 and 7, whereas Fe and Mn have a better availability at a pH of below 6, but Ca and Mg at a pH above 6.5. The availability of the transition metals in soils to plants can be limited significantly by the formation of various complexes. Oxides and oxyhydroxides of Fe, Zn, Cu and Mn play an important role in the solubility of these elements in the soil, the prevailing redox potential of the particular soil structure playing a decisive role.
There are several examples of biochemistry in which metallic trace elements in specific functions and in particular unique metal combinations transform enormous masses in the biosphere. For example, a combination of manganese and magnesium and their stoichiometric ratio play an important role in photosynthesis processes. The ionic ratios of molybdenum and magnesium or copper and magnesium play an important role in substrate oxidation and subsequent energy storage in digestion processes.
Although iron is one of the most frequently occurring elements in the soil, the quantities available to plants are limited in many soils. Iron from most primary soil minerals is present in the divalent Fe(II) form, but is oxidized by weathering processes under aerobic conditions and is fixed as Fe(III) oxide. Although Fe(III) is present in soils in considerably larger quantities, Fe(II) is of greater importance in plant physiology, since it is by far the more preferred form of uptake. Fe(II) from conventional fertilizers also oxidizes rapidly and often precipitates in the soil matrix in unused form. Known fertilizers comprise iron in the form of iron sulphate, iron ammonium sulphate, iron ammonium citrate, iron gluconate, iron ligninsulphonates or also in chelated form. Iron sulphate, for example, has an iron content of about 20%, is relatively inexpensive as a fertilizer and can be applied as soil application or as leaf application. However, iron sulphate applied by soil application is often ineffective because of rapidly occurring precipitation reactions to give Fe(III), in particular at soil pH values above 7.
Leaf application of trace elements can be improved by surface-active agents, which have an effect on the distribution and uptake of the nutrients. Surface-active agents which are employed are plant oils, rape methyl ester, protein derivatives, ionic and nonionic wetting agents, organosilicones, polymers, waxes etc. They act as wetting agents for an improved wetting of the leaf surface, as penetration agents to facilitate uptake of the nutrients into the plant or as adhesives for improving the adhesion of the nutrients to the leaf surface.
Iron sulphate and iron chelates with DTPA or EDTA are employed via leaf application to eliminate or prevent chloroses, but with varying results. While iron sulphate is the less expensive compound, the more expensive iron chelates often show a better action because the iron is present in stabilized form and furthermore can also be taken up directly by the leaf. Attempts to improve the uptake of iron via a leaf application include formulation of the iron sulphate together with citrate or the direct use of iron lignosulphonates and Fe(III) salts under acid formulation conditions.
Iron chelates applied to the soil are water-soluble and easily washed out of the root zone of the plants in the event of intensive irrigation or during the period of low vegetation in autumn and winter. A possibly underestimated problem of some synthetic chelates is the potential bonding capacity of heavy metals with a subsequently increased washing out. Some iron chelates presumably have an adverse effect on microorganisms and mycorrhizae present in the soil.
To improve the efficacy of less expensive iron fertilizer forms, such as, for example, iron sulphate, so-called controlled-release fertilizers (CRF) having a defined slow release of nutrients have been proposed. Other approaches relate to a band application of iron sulphate with hydrophilic polyacrylamide gels, with iron fertilizer granules enveloped in sulphur or the immobilization of iron chelates in Sepharose gel. Various naturally occurring crystalline iron compounds, such as vivianite (Fe3(PO4)2.8H2O) and pyrites (FeS2), show a higher effectiveness than FeSO4, but are less available and therefore more expensive.
The preparation of synthetic vivianite is described as relatively favourable and simple, in that iron sulphate heptahydrate and mono- or diammonium phosphate are simultaneously dissolved in water directly on site by the end user. The product is an initially white suspension which, however, rapidly assumes a greenish-blue colour, which is characteristic of partially oxidized vivianite. In order to prevent the vivianite particles, which have a size of about 2-10 μm, from settling on the base of the preparation container, the suspension must be stirred continuously and employed as quickly as possible.
Many references to synergistic effects of various nutrient elements on plants, in particular with simultaneous application in the vicinity of the rhizosphere, are known to the person skilled in the art. For example, ammonium, sulphate or potassium are said to increase the availability of iron in lime-containing soils significantly due to the physiological acidification of the rhizosphere.
An adequate supply of trace elements to agricultural and horticultural crops is of decisive importance to the nutrition of humans and animals. Research is focussed on the concentrations of iron and zinc in plant foodstuffs, such as cereals and rice, and the bioavailability thereof. It is known that the trace element concentrations in cereals in particular differ significantly and can be increased by known measures.
Nutrient additives for animal nutrition are said to improve the quality of the feed and the health and output of the animals. Animals kept agriculturally meet the majority of their trace elements requirement via the plant food offered to them. The presence of important trace elements in vegetative plant parts and plant seeds is therefore of the greatest importance for animal nutrition.
As in plant nutrition, in animal nutrition there are also known antagonistic interactions between trace elements in the organism. One of the certainly best-researched interactions concerns the antagonistic relationship of the trace elements copper, molybdenum and sulphur. An excess of sulphur, molybdenum and iron in the diet is said to impair the uptake and utilization of copper. This leads to deficiency symptoms, even in the case of adequate copper concentrations in the diet. There is therefore the need to coordinate animal nutrition with respect to the concentration ratios of the trace nutrients.
The provision of nutrient substances in a suitable composition is also of decisive importance for the microorganisms employed in industrial microbiological (biotechnology) processes in order to optimize the productivity of the particular system.
The fermentation, i.e. the breakdown metabolism of organic matter by microorganisms under either aerobic or anaerobic conditions, delivers diverse end products. In this context, in addition to essential further process parameters, such as temperature, pH etc., the optimum nutrient composition of the medium is of decisive importance for success. Depending on the use, the important nutrients can also include essential trace elements, such as Cu, Co, Fe, Mn, Mo or Zn.